fix: mock provider returns today's date in classify response

Without a date, articles are routed to "Articles sans date" instead
of their classified category, breaking pipeline tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/services/llm/mock.rs

@@ -58,10 +58,12 @@ impl LlmProvider for MockLlmProvider {
                 .map(|l| l.trim_start_matches("Titre : ").to_string())
                 .unwrap_or_else(|| "Mock Article".to_string());
 
+            let today = chrono::Utc::now().format("%Y-%m-%d").to_string();
             return Ok(json!({
                 "title": title,
                 "summary": format!("Mock summary for: {}", title),
                 "category": self.default_category,
+                "date": today,
                 "is_article": true,
             }));
         }
@@ -96,6 +98,7 @@ mod tests {
         assert_eq!(result["title"], "GPT-7");
         assert_eq!(result["category"], "AI News");
         assert_eq!(result["is_article"], true);
+        assert!(result["date"].as_str().is_some_and(|d| !d.is_empty()), "date should be set");
     }
 
     #[tokio::test]